Modern, complex materials, including (but not limited to) composite and multilayer materials, are prone to defects that cannot be seen from the outside. Some of the most effective methods of detecting these defects are ultrasonic imaging, imaged shearography, and thermal shearography. Ultrasonic imaging is limited by the fact that increasing the frequency of the ultrasonic signal improves imaging resolution, but also reduces the propagation loss. In addition, the higher the ultrasonic frequency, the greater the loss of signal strength when crossing a material boundary. Imaged shearography can produce a high-resolution scan of the material, but its effectiveness decreases rapidly with depth of the defect to be detected. However, many defects are not seen in this technique.
Thermal shearography permits defect detection at greater depth, but the lack of direction in the thermal stress makes it much more difficult to locate the defect that has been detected. In addition, slow propagation of thermal energy makes the detection and location process very slow.
Accordingly, a rapid technique of detecting and localizing buried defects is needed.